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Ill admit Im a bit of a vitamin D skeptic. Studies demonstrating that the wonder-vitamin can improve cognition, decrease the risk of colon cancer, and prevent heart disease are often observational in nature. These associations are always confounded by sunlight exposure and diet  two factors which themselves are strongly associated with a variety of health outcomes. It's no surprise that randomizedtrials of vitamin D supplementation have been less than impressive.

So a study, appearing in PLOS medicine, linking lower Vitamin D levels to the development of multiple sclerosis caught my eye.

You want to know if some biomarker (like vitamin D level) is causally linked to a disease (like MS). Vitamin D level is determined by a slew of things that you have no control over, like sun exposure, but there may be genetic polymorphisms that predispose you to lower-than-average vitamin D levels for your entire life. At birth, you may be "randomized" to one of these genes. If low vitamin D causes MS, than surely people who inherited those low-vitamin D genes would have a higher risk of MS.

The caveat is that those genes have to be linked to MS only through a Vitamin D pathway  you want to avoid what's called "pleiotropy". And those genes need to not be near any other genes on the chromosome that can cause MS. Finally, the genes need to be randomly spread through the population of interest  if they are preferentially carried by a certain ethnic group you might be finding a marker of increased risk in that group due to cultural, environmental, or other biologic factors.

Point is  Mendelian randomization is hard. Did the authors make the grade?

First they identified 4 single-nucleotide polymorphisms (SNPs) that were associated with low Vitamin D levels from a database of over 30,000 individuals. These variants were all in genes related to Vitamin D synthesis or metabolism. They then looked for these SNPs in a huge genetic study of MS patients comprising 14,000 cases and 24,000 controls.

The big finding is that, yes, people with MS were more likely to carry at least one of those low vitamin D genes. And while the genes weren't obviously associated with MS risk factors outside of Vitamin D, they were involved in things like steroid synthesis, so the potential for unknown off-target effects is pretty high.

Best we can tell, the implication is that increasing your vitamin D level significantly  by say 10  50 nmol/L  may decrease your risk of MS by up to 50%. Clearly, these numbers will have to be borne out in clinical trials. But considering that genetically low vitamin D is a lifetime risk, my hope that a short-term vitamin D supplementation trial will show a positive effect is very slim. There are ongoing efforts to start a Vitamin D trial among individuals with a first MS flare  but that is clearly not the population who was studied here.

In the end, this study ends up being long on methods, and short on actionable results.